Effects of flow velocity and settlement location on growth rates of early juveniles of the pedunculate barnacle Pollicipes polymerus Sowerby, 1833

The intertidal lepadomorph barnacle Pollicipes polymerus lives gregariously, preferentially settling on conspecifics. This study asks if and how P . polymerus individuals gain in overall fitness from gregarious settlement. The study addresses two questions: 1) Is the growth rate of early juveniles dependent on the velocity of the water in which they feed? and 2) Does settlement on conspecifics benefit early juveniles by increasing growth rate? Two treatment groups (solitary and gregarious) and an experimental velocity gradient were studied in marina and tank environments, with P . polymerus individuals ranging in size from 1 to 3 mm (measured by rostro- carinal length). For the trials carried out in a marina environment, growth rates ranged from 0.1 to 1.2 mm per week at any given velocity. For the trials in a tank environment, individual growth never exceeded 0.6 mm per week. Overall results indicate that neither current velocity nor settlement location had consistent effects on early juvenile growth rates. In the tank environment, where food concentrations were lower, velocity had a significant negative effect on the growth rate of gregarious but not solitary juveniles. Meanwhile, settlement on a conspecific had a significant positive effect on average juvenile growth in one of the four trials. The absence of consistent trends within and across trials indicates that velocity and settlement location for early juveniles may not be primary factors in gregarious settlement of this barnacle. The results also have important implications for aquaculture; they suggest that P. polymerus juveniles do not require specific flow speeds or adult substrata to cultivate newly-settled barnacles